Telephony is increasingly "feature rich," i.e., there is an ever increasing array of features and services available to both business and individual customers. Features are being implemented in "smart" telephones, in local switching systems, and in long distance network switching systems to provide customers with conveniences and facilities that were unavailable until recent times. Three-way calling, call waiting, speed calling, credit card calling, etc., are now familiar examples of switch-implemented features. "Smart" telephones, facsimile machines and the like can automatically redial busy or no-answer numbers. All such features and services require that the device providing the feature recognize audible signaling tones, such as dual-tone, multi-frequency (DTMF) digits and call progress tones, which are generated by the network or local switching system.
Detection of audible signaling tones reliably is a well known problem in the art. For example, Cave et al., U.S. Pat. No. 4,405,833 which issued to TBS International, Inc., on Sep. 20, 1983, describes a telephone call progress tone and answer identification circuit. In Cave, call progress tones are identified by measuring the periods of the envelope cycles in the frequency range of DTMF and call progress tones. The periods are classified by their lengths, and compared to known periods as used by the various operating companies. This method requires many periods in order to determine a tone, and is subject to false positives, or "talk off" due to noise or verbal patterns of individuals which may resemble a call progress tone when analyzed in this fashion. Talk off can occur when speech, music or other sounds exist in the signal wherein the frequency and amplitude of the fundamentals or harmonics resemble that of audible signaling tones. Most signal processors have similar complexity and talk off problems.
The problems of detecting audible signaling tones arise from the nature of the telephone network. The frequencies for DTMF digits and call progress tones were selected so that they were near the lower end, but well within the telephone band pass of 180-3600 Hz. DTMF digits are defined by the following matrix:
TABLE 1 ______________________________________ 1209 Hz 1336 Hz 1477 Hz 1633 Hz ______________________________________ 697 Hz 1 2 3 A 770 Hz 4 5 6 B 852 Hz 7 8 9 C 941 Hz * 0 # D ______________________________________
Call progress tones are defined by the following frequency matrix:
TABLE 2 ______________________________________ 0 Hz 440 Hz 480 Hz 620 Hz ______________________________________ 350 Hz Dial Tone/ Stutter Dial Tone 440 Hz Call Waiting/ Ringing Variations 480 Hz Busy/ Reorder ______________________________________
These two matrixes are well known in the art, wherein each digit or call progress tone is defined by the intersection of the horizontal and vertical frequency. When the two frequencies are simultaneously received, it is universally interpreted to the alphanumeric character or the call progress tone as shown at the matrix intersections. Hence indicators used in the telephone system, such as DTMF digits and call progress tones, each correspond uniquely to a DTMF tone defined by two specified frequency components. Each tone in the above tables has a predefined functional meaning accepted in the art.
A call waiting tone is a special case of a DTMF tone. Call waiting is defined similarly to the other DTMF and call progress tones, but has one frequency defined as 0 Hz. In actuality, there is no component at 0 Hz, as there is no D.C. component at 0 Hz for call waiting tones. As a consequence of having one frequency which is shared with Ringing, call waiting tones are harder to detect reliability without false positives.
Additionally, most call progress tones have at least two different rhythmic patterns or cadences with unique meanings. "Busy," for example, may be fast busy (reorder) or slow busy (busy line). Dial tone may be stutter dial tone (short tones and a long tone), as used in features such as three-way calling, or as a constant tone for regular dial tone. Call waiting can have several different patterns, used, for example, when multiple telephone numbers are associated with one telephone to distinguish the line that has the call waiting.
Dual-frequencies were selected for the majority of these tones so that they are not harmonically related, and so that there is little similarity to the harmonically complex relationships of the human voice. However, human speech includes many fundamental and harmonic frequencies in the 350-1633 Hz range of DTMF digits and call progress tones, which cause talk off. Noise on the line compounds the false positive problem because noise occurs in all frequency bands.
Additionally, when two frequencies of a DTMF digit or call progress tone are transmitted, their relative amplitudes ideally should be identical. Due to the nature of the telephone network, however, the relative amplitudes are rarely the same. Therefore, the relative amplitudes of the two frequencies of a specific digit or tone must be within a predefined ratio or "twist." Since amplitude typically decreases as the frequency increases on a telephone line, the "twist" for DTMF and call progress tones is generally accepted at 8 dB. Occasionally the reverse occurs, and the higher frequency has a greater amplitude than the lower frequency. This is called "reverse twist" and the generally accepted "reverse twist" ratio is 4 dB. If the twist or reverse twist is greater than these tolerances, the tone should not be recognized as a DTMF digit or call progress tone. Finally, each DTMF digit must be present for at least 40 ms. to be recognized.
Therefore, a problem in the art is that there is no method and apparatus for reliably detecting audible signaling tones in both the presence and absence of speech. A further problem in the art is that there is no method and apparatus for reliably detecting call waiting tones in both the presence and absence of speech.